Brendan Eich's @media keynote is worth a look if you are interested in ES4/JS2 (via henso). I found a reference to work done by Michael Franz's team at UCI especially interesting: Chang et al. (2007) - "Efficient Just-In-Time Execution of Dynamically Typed Languages Via Code Specialization Using Precise Runtime Type Inference". From the abstract:

"Dynamically typed languages such as JavaScript present a challenge to just-in-time compilers. In contrast to statically typed languages such as JVML, in which there are specific opcodes for common operations on primitive types [..], all operations in dynamically typed language such as JavaScript are late-bound. [..] We present a new approach to compiling dynamically typed languages in which code traces observed during execution are dynamically specialized for each actually observed run-time type. For most benchmark programs, our prototype JavaScript virtual machine outperforms every other JavaScript platform known to us."